Portable communication appliances, WLAN routers, etc., or more generally: transmission/reception devices that communicate by means of RF signals require an apparatus that separates the transmission signals and received signals from one another. In general, transmission signals are much stronger than received signals, which means that protection of the received signal path forms an important aspect of this apparatus. Duplexers are one way of implementing such separating apparatuses. In general, duplexers are in this case limited to frequency bands stipulated in advance. However, particularly against the background of the now large number of different frequency bands, it would be advantageous if a single duplexer could serve a plurality of frequency bands.
In this case, the duplexer is intended to perform the task of distributing the signals between a chipset and any filters that are still present. The circuit complexity should be as low as possible. The duplexer should be compatible with a multiplicity of different filter technologies, have a small physical size and, in particular, allow a high level of isolation between transmission signals and received signals.
Previous solutions for these requirements are essentially based on extending known duplexer circuits by tunable impedance elements, or on the use of switches by means of which filter elements are additionally connectable to a filter topology.
As such, the article “Reconfigurable Multi-band SAW Filters For LTE Applications”, Xiao Ming et al., Power Amplifiers For Wireless And Radio Applications (PAWR), 2013 IEEE Topical Conference, Jan. 20, 2013, pages 82-84, discloses essentially conventional RF filters that are reconfigurable by means of switches. Filters reconfigurable by means of switches do not allow continuously tunable duplexers in this case, however.
The article “Tunable Filters Using Wideband Elastic Resonators”, Kadota et al., IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 60, No. 10, October 2013, pages 2129-2136, discloses filter circuits in which tunable capacitors are added to RF filters with acoustic resonators.
The article “A Novel Tunable Filter Enabling Both Center Frequency and Bandwidth Tunability”, Inoue et al., Proceedings of the 42nd European Microwave Conference, Oct. 29-Nov. 1, 2012, Amsterdam, the Netherlands, pages 269-272, discloses RF filters with tunable capacitors and tunable inductances.
The article “RF MEMS-based Tunable Filters”, Brank et al., 2001, John Wiley & Sons, Inc. Int J RF and Microwave CAE11: pages 276-284, 2001, discloses interconnections comprising L and C elements, wherein the capacitances of the capacitive elements are variable.
The article “Design of a Tunable Bandpass Filter with the Assistance of Modified Parallel Coupled Lines”, Tseng et al., 978-1-4673-2141-9/13/$31.00, 2013 IEEE, discloses tunable filters with coupled transmission lines.
The article “Tunable Isolator Using Variable Capacitor for Multi-band System”, Wada et al., 978-1-4673-2141-9/13/$31.00, 2013 IEEE MTT-S Symposium and the published specification WO02012/020613 disclose the use of isolators in RF filters.
In summary, it can essentially be stated for the RF circuits known from the aforementioned articles that known filter topologies tunable filter circuits are obtained by virtue of the addition of variable elements, e.g., switches or variable impedance elements. The problem with this is that the known filter topologies used are essentially optimized for the use of impedance elements with constant impedance. Although tunable filters are made possible, performance suffers as a result of the tunability.